Method of forming a thermal barrier coating system with engineered surface roughness

ABSTRACT

A method of manufacturing a substrate ( 16 ) with a ceramic thermal barrier coating ( 28, 32 ). The interface between layers of the coating contains an engineered surface roughness ( 12, 24 ) to enhance the mechanical integrity of the bond there between. The surface roughness is formed in a surface of a mold ( 10,20 ) and is infused by a subsequently cast layer of material ( 16, 28 ). The substrate may be partially sintered ( 76 ) prior to application of the coating layer(s) and the coated substrate and coating layer(s) may be co-sintered to form a fully coherent strain-free interlayer.

This application is a continuation-in-part of co-pending applicationSer. No. 13/221,077 filed 30 Aug. 2011 which is incorporated byreference herein in its entirety.

FIELD OF THE INVENTION

This invention relates generally to ceramic coated metal components andto methods for applying such coatings.

BACKGROUND OF THE INVENTION

It is known to use ceramic thermal barrier coatings to protect metallicparts that are exposed to hot combustion gas in a gas turbine engine.United States Patent Application Publication US 2009/0110954 A1describes known thermal barrier coating systems which typically includea bond coat material deposited between the ceramic thermal barriercoating and the underlying metal substrate. It is also known thatimproved adherence of the thermal barrier coating can be achieved byproviding a roughened surface on the bond coat material, such as bycontrolling the process parameters used to deposit the bond coatmaterial. One such technique is described in United States PatentApplication Publication US 2010/0092662 A1.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in the following description in view of thedrawings that show:

FIGS. 1 through 5 illustrate steps in a method in accordance with anembodiment of the invention.

FIG. 6 is a cross sectional view of a gas turbine component inaccordance with an embodiment of the invention.

FIG. 7 illustrates the steps in a method in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present inventors have recognized a need for further improvements intechniques for enhancing the adhesion of a ceramic thermal barriercoating. For example, while it is known to affect the surface roughnessof a bond coat material by controlling the spray parameters by which thematerial is deposited, the present inventors have found that such sprayprocess controls may be inadequate for some advanced gas turbine engineapplications due to variability in the structure of the mechanicalinterface between layers in the thermal barrier coating system overmultiple applications of a process.

FIGS. 1-6 illustrate the steps of a method in accordance with oneembodiment of the present invention. In FIG. 1, a mold 10 (substratemold) is formed to have a first designed surface roughness 12 on aninterior surface 14. The surface roughness 12 is not a randomly derivedtopography, such as might be achieved through a sand blasting orspraying process, but rather is an engineered surface that isspecifically designed to have desired surface feature geometries andsizes. Such designed surface topographies may be formed by atomo-lithographic process, as described in U.S. Pat. No. 7,141,812, orby any other known process. This substrate mold is used to cast a greenbody 16 as shown in FIG. 2, thereby replicating the designed surfaceroughness 12 onto an exterior surface 18 of the green body 16. The greenbody 16 may be cast from a ceramic or metal powder slurry, for example,and may be in the shape of a substrate of a component for a gas turbineengine.

Another mold 20 (bond layer mold) is formed to have another designedsurface roughness 22 on an interior surface 24 as illustrated in FIG. 3.Here, again, the surface roughness 22 is not a randomly derivedtopography, but rather is an engineered surface that is specificallydesigned to have desired surface feature geometries and sizes. The greenbody 16 is removed from the mold 10 and is positioned within mold 20with a small controlled space 26 separating the green body surface 18from the second mold surface 24. The space 26 represents a desiredthickness of a bond coat material to be joined to the green body 16.

A bond coat material 28 is then cast in slurry form into the space 26and allowed to solidify as illustrated in FIG. 4. The slurry cooperateswith the surface roughness 12 on the outer surface 18 of the green body16 to form a desired mechanical interconnection between the green body16 and the bond coat material 28. Furthermore, the surface roughness 22on the interior surface 24 of the mold 20 is transferred into the bondcoat material 28, such that when the green body 16 with coating 28 isremoved from the mold 20, its outer surface 30 is available forreceiving a thermal barrier coating material 32 to form a thermallyinsulated component 34 as shown in FIG. 5. The thermal barrier coatingmaterial 32 may be applied by a known spray process or it may be cast byusing yet another mold 36 (coating mold).

Advantageously, a thermally insulated component according to anembodiment of this invention has a first desired mechanicalinterconnection between a substrate and a bond coating that is definedby a first designed surface roughness formed on the substrate, and asecond desired mechanical interconnection between the bond coating andan overlying ceramic thermal barrier coating that is defined by a seconddesigned surface roughness formed on the bond coat material. The firstand second mechanical interconnections may have different physicalparameters as may be desired by the designer. For example, thedimensions of the roughness features may be designed to be differentbetween the two mechanical interconnections in response to differencesin the physical parameters of the two different slurries used to castthe green body 16 and the bond coat material 28. Furthermore, thephysical parameters of the first and/or second mechanicalinterconnections, and the thickness of the bond coat material may varyfrom one region of the component to another. For example, a leading edgeof an airfoil component may be subjected to more severe impact damagethan the remainder of the airfoil during operation of a gas turbineengine. That airfoil manufactured in accordance with the presentinvention may have a thicker layer of the bond coat material in theleading edge area and/or it may have a mechanical interconnection in theleading edge area that provides more surface area contact between thetwo material layers (i.e. a more aggressive surface roughness pattern).

FIG. 6 is an illustration of one such thermally insulated component 40in accordance with an embodiment of the invention. The component 40includes a substrate 42 protected by a thermal barrier coating 44 whichvaries from one region of the component to another. The thermal barriercoating 44 includes a layer of bond coat material 46 and a top layer ofceramic insulating material 48. In a first region 50 of the component40, such as a suction side of an airfoil or a straight region of acombustor transition piece, the mechanical interconnection 52 betweenthe substrate 42 and the bond coat material 46 may be created by aroughness of the substrate surface approximating a sine wave shape witha relatively long period; whereas in a second region 54 of the component40, such as an airfoil leading edge or a curved region of a combustortransition piece, the mechanical interconnection 56 between thesubstrate 42 and the bond coat material 46 may be created by a roughnessof the substrate surface approximating a sine wave shape with arelatively shorter period. The relatively shorter sine wave shapeprovides a more aggressive interconnection with more contact area perunit area of surface. Furthermore, the mechanical interconnection 58between the bond coat material 46 and the ceramic insulating material 48in the first region 50 may be created by a roughness of the bond coatsurface characterized by saw tooth shapes 64; whereas the mechanicalinterconnection 62 between the bond coat material 46 and the ceramicinsulating material 48 in the second region 54 may be created by aroughness of the bond coat surface including protruding undercut shapes60. The protruding undercut shapes 60 provide a more aggressiveinterconnection than do the saw tooth shapes 64. Furthermore, theaverage thickness of the bond coat material 46 may be greater in region54 than in region 50. Such features may be produced with precision inrepeated applications by the molding and casting techniques describedabove and illustrated in FIGS. 1-5. Thus, the present invention providesdegrees of flexibility and precision of control in the design of thermalbarrier coating systems that are not available with prior arttechniques.

A primary purpose for utilizing a bond coat layer in prior art ceramicthermal barrier systems is to provide a desired degree of roughness inthe surface forming the metal-to-ceramic interface, since the castmetallic substrate surface would not provide a desired degree ofmechanical interface with the ceramic insulating layer if the bond coatlayer were not present. Furthermore, traditional MCrAlY bond coatmaterials also provide a supply of aluminum for the formation of aprotective alumina layer when the component is exposed to hightemperatures. In one embodiment of the present invention, the green body16 of FIG. 2 is cast using an alumina-forming substrate material with adesired engineered surface roughness 12 appropriate for the directapplication of the ceramic thermal barrier coating material 32 withoutany intervening bond coat layer. The surface roughness 12 in suchembodiments may correspond to or improve upon the surface roughness thatis achieved with the traditional thermally sprayed bond coat material.Thus, in some embodiments, the present invention provides a desireddegree of roughness at the surface of the substrate effective to ensurean adequate metal-to-ceramic interface without the need for a bond coatlayer.

The present invention is advantageously implemented with a processwherein the metal and ceramic materials are selected and processed to becooperatively matched for both sintering shrinkage and thermal expansionperformance. One such process 70 is illustrated in FIG. 7 which includesthe steps of: 72—selecting CTE-compatible metal and ceramic materials;74—forming a substrate from a powder of the metal material; 76—partiallysintering the substrate; 78—forming a layer on the substrate from apowder of the ceramic material containing a quantity of nano-particleseffective to suppress a sintering temperature of the material; and79—co-sintering the substrate and the layer of ceramic material to finaldensity. The step 76 of partially sintering the substrate may ensurethat the shrinkage of the substrate and the layer during theco-sintering step are approximately the same, and the step 78 ofincluding a quantity of nano-particles may ensure that the sinteringtemperatures of the substrate and the layer are approximately the sameto enable the co-sintering step 79.

One will appreciate that to achieve a desired mechanical interfacebetween the layers, the step 76 of forming the substrate may beaccomplished in accordance with the molding process described above withrespect to FIGS. 1 and 2, and the step 78 of forming a layer on thesubstrate may be accomplished in accordance with the molding processdescribed above with respect to FIGS. 3 and 4. The result is a layeredmaterial system having a fully coherent strain-free interlayerconsisting of interspersed elements of both constituents along aninterface defined by an engineered surface roughness topography. Theresulting co-processed system is dense and dimensionally stable and maybe used in advanced modular inserts for aggressive, impact resistant,high temperature gas turbine applications. In various embodiments, themethods disclosed herein permit the co-processing of a low expansionalloyed refractory metal system based on chromium, molybdenum, niobium,tantalum, tungsten and/or iron with a sinter-active ceramic powderthermal barrier overlay composition employing a bi-modal particle sizedistribution of alumina, stabilized zirconia and/or yttrium aluminumgarnet powders.

The processes and materials described herein allow a much thickerceramic layer on a metal substrate than was previously possible withoutthe use of a flexible intermediate layer and/or engineered slots in theceramic layer for strain relief. Whereas prior monolithic ceramic layersin this temperature range were limited to about 0.3 mm thick, thepresent invention can produce durable monolithic ceramic layers over 1.0mm thick, including over 2.0 mm thick, for example up to 3.0 mm thick insome embodiments, on superalloy substrates for use over a wide operatingtemperature range such as 0-1000° C. or 0-1500° C. in some embodiments.Herein, “monolithic” means a layer without a flexible intermediate layeror engineered slots for strain relief.

While various embodiments of the present invention have been shown anddescribed herein, it will be obvious that such embodiments are providedby way of example only. Numerous variations, changes and substitutionsmay be made without departing from the invention herein. Accordingly, itis intended that the invention be limited only by the spirit and scopeof the appended claims.

What is claimed is:
 1. A thermally insulated component comprising: agreen body comprising first and second major sides, which are oppositeone another, first and second minor sides, which are opposite oneanother and which extend between the first and second major sides, and afirst powder, the green body defining, along the first major side andthe first and second minor sides, a first surface that is integral tothe green body, substantially surrounds and extends along only the firstmajor side and the first and second minor sides of the green body anddefines first and second engineered surface roughness characteristicswithin first and second regions of the first surface, respectively, thefirst engineered surface roughness characteristic having a mechanicalinterconnection having a plurality of protruding undercut shapes; and amonolithic ceramic insulating layer that comprises a second powder,coats the first surface and defines a second surface, wherein theprotruding undercut shapes extend into the monolithic ceramic insulatinglayer and the second surface defines third and fourth engineered surfaceroughness characteristics within first and second regions of the secondsurface, respectively, and each protruding undercut shape comprises: aplanar surface; a boss extending normally from a proximal end thereof atthe planar surface to a distal end thereof remote from the planarsurface; and a hook element extending perpendicularly from the distalend of the boss, the hook element having a length which is perpendicularto a length of the boss, from the proximal end thereof to the distal endthereof, which traverses an entirety of the distal end of the boss andprotrudes from a plane of a side of the boss, and which is at least aslong as the length of the boss.
 2. The thermally insulated component ofclaim 1, wherein: the first powder is alumina-forming.
 3. The thermallyinsulated component of claim 1, wherein: the first surface is asubstantially spatially inverted replica of a substrate mold surfacehaving a tomo-lithographically-formed surface topography.
 4. Thethermally insulated component of claim 1, wherein: the first surface isa substantially spatially inverted replica of a substrate mold surfacethat comprises a surface topography that comprises protruding undercuts.5. The thermally insulated component of claim 1, wherein: the firstengineered surface roughness characteristic facilitates directapplication of the monolithic ceramic insulating layer onto the firstsurface.
 6. The thermally insulated component of claim 1, furthercomprising: a sufficient number of nanoparticles in the second powder toreduce a co-sintering temperature of the first powder to the secondpowder.
 7. The thermally insulated component of claim 1, wherein: ashrinkage of the first powder during co-sintering and a shrinkage of thesecond powder during the co-sintering are approximately identical. 8.The thermally insulated component of claim 1, wherein: a sinteringtemperature of the first powder and a sintering temperature of thesecond powder are approximately identical.
 9. The thermally insulatedcomponent of claim 1, wherein: the ceramic insulating material employs abi-modal particle size distribution of alumina, stabilized zirconia,and/or yttrium aluminum garnet powders.
 10. The thermally insulatedcomponent of claim 1, wherein: the ceramic insulating material defines amonolithic ceramic insulating layer that has a thickness betweenapproximately 1.0 mm and approximately 3.0 mm.
 11. A thermally insulatedcomponent comprising: a green body comprising first and second majorsides, which are opposite one another, first and second minor sides,which are opposite one another and which extend between the first andsecond major sides, and a first sintered powder, the green bodydefining, along the first major side and the first and second minorsides, a first surface that is integral to the green body, substantiallysurrounds and extends along only the first major side and the first andsecond minor sides of the green body and defines first and secondengineered surface roughness characteristics within first and secondregions of the first surface, respectively, the first engineered surfaceroughness characteristic having a mechanical interconnection having aplurality of protruding undercut shapes; and a monolithic ceramicinsulating layer that comprises a second sintered powder, coats thefirst surface and defines a second surface, wherein the protrudingundercut shapes extend into the monolithic ceramic insulating layer, thesecond surface defines third and fourth engineered surface roughnesscharacteristics within first and second regions of the second surfaceand a first portion of the first sintered powder is co-sintered to afirst portion of the second sintered powder, and each protrudingundercut shape comprises: a planar surface; a boss extending normallyfrom the planar surface; and a hook element extending perpendicularlyfrom a distal end of the boss, the hook element having a length which isat least as long as a length of the boss.
 12. The thermally insulatedcomponent of claim 1, wherein the second engineered surface roughnesscharacteristic is different from the first engineered surface roughnesscharacteristic.
 13. The thermally insulated component of claim 12,wherein the third and fourth engineered surface roughnesscharacteristics are both different from the first and second engineeredsurface roughness characteristics.
 14. The thermally insulated componentof claim 1, wherein the second engineered surface roughnesscharacteristic has saw toothed shapes, the third engineered surfaceroughness characteristic has wavy shapes with a first periodicity andthe fourth engineered surface roughness characteristic has wavy shapeswith a second periodicity.